[dm-devel] [PATCH 4/4] Support discard if at least one underlying device supports it

[Mike Snitzer]

On Fri, Jul 02 2010 at  4:00pm -0400,
Mikulas Patocka <mpatocka at redhat.com> wrote:

> 
> 
> On Fri, 2 Jul 2010, Mikulas Patocka wrote:
> 
> > > As we discussed, we have a challenge where we need DM to avoid issuing
> > > a barrier before the discard IFF a target doesn't support the discard
> > > (which the barrier is paired with).
> > > 
> > > My understanding is that blkdev_issue_discard() only cares if the
> > > discard was supported.  Barrier is used just to decorate the discard
> > > (for correctness).  So by returning -EOPNOTSUPP we're saying the discard
> > > isn't supported; we're not making any claims about the implict barrier,
> > > so best to avoid the barrier entirely.
> > > 
> > > Otherwise we'll be issuing unnecessary barriers (and associated
> > > performance loss).
> > > 
> > > So yet another TODO item... Anyway:
> > > 
> > > Acked-by: Mike Snitzer <snitzer at redhat.com>
> > 
> > Unnecessary barriers are issued anyway. With each freed extent.
> > 
> > The code must issue a "SYNCHRONIZE CACHE" to flush cache for previous 
> > writes, then "UNMAP" and then another "SYNCHRONIZE CACHE" to commit that 
> > unmap to disk. And this in loop for all extents in 
> > "release_blocks_on_commit".
> > 
> > One idea behind "discard barriers" was to submit a discard request and not 
> > wait for it. Then the request would need a barrier so that it doesn't get 
> > reordered with further writes (that may potentially write to the same area 
> > as the discarded area). But discard isn't used this way anyway, 
> > sb_issue_discard waits for completion, so the barrier isn't needed.
> > 
> > Even if ext4 developers wanted asynchronous discard requests, they should 
> > fire all the discards at once and then submit one zero-sized barrier. Not 
> > barrier with each discard request.
> > 
> > This is up to ext4 developers to optimize and remove the barriers and we 
> > can't do anything with it. Just send "SYNCHRONIZE 
> > CACHE"+"UNMAP"+"SYNCHRONIZE CACHE" like the barrier specification wants...
> > 
> > Mikulas
> 
> BTW. I understand that the current dm implementation will send two useless 
> consecutive "SYNCHRONIZE CACHE" commands discard is directed to the part 
> of the device that doesn't support it.

Issue 1 ^^^

> But the problem is that when you use discard on a part of the device that 
> supports discard, it also sends two useless "SYNCHRONIZE CACHE" commands 
> --- they are useless for functionality, but mandated by the barrier 
> specification.

Issue 2 ^^^

Those are 2 different issues.  Please don't join them as if they are one
in the same.  DM should treat a discard as a first class request (which
may or may not have a barrier).  If a region doesn't support the discard
DM has no business processing anything related to the discard (barriers
included).  It is as simple as that.

> The fix is supposedly this:
> 
> ---
>  include/linux/blkdev.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux-2.6.35-rc3-fast/include/linux/blkdev.h
> ===================================================================
> --- linux-2.6.35-rc3-fast.orig/include/linux/blkdev.h	2010-07-02 21:59:21.000000000 +0200
> +++ linux-2.6.35-rc3-fast/include/linux/blkdev.h	2010-07-02 21:59:37.000000000 +0200
> @@ -1021,7 +1021,7 @@ static inline int sb_issue_discard(struc
>  	block <<= (sb->s_blocksize_bits - 9);
>  	nr_blocks <<= (sb->s_blocksize_bits - 9);
>  	return blkdev_issue_discard(sb->s_bdev, block, nr_blocks, GFP_KERNEL,
> -				   BLKDEV_IFL_WAIT | BLKDEV_IFL_BARRIER);
> +				   BLKDEV_IFL_WAIT);
>  }
>  
>  extern int blk_verify_command(unsigned char *cmd, fmode_t has_write_perm);

Hmm, older kernels use DISCARD_FL_BARRIER which merely mapped to
BLKDEV_IFL_BARRIER.

Seems you've stumbled onto a bug in the conversion that commit
"blkdev: generalize flags for blkdev_issue_fn functions"
(fbd9b09a177a481eda) performed?

That commit seems to have incorrectly replaced DISCARD_FL_BARRIER with
both: BLKDEV_IFL_WAIT | BLKDEV_IFL_BARRIER

Dmitry and/or Jens was this intended?

Mike